Avalanche photodiode structure

ABSTRACT

An avalanche photodiode (APD) includes an anode layer, a cathode layer, an absorption layer between the anode layer and the cathode layer, a first multiplying stage between the absorption layer and the cathode layer, a second multiplying stage between the first multiplying stage and the cathode layer, and a carrier relaxation region between the first and second multiplying stages. Each multiplying stage includes, in the direction of drift of electrons, a first layer that is doped with acceptors, a second layer that is substantially undoped, a third layer that is doped with acceptors, a fourth layer that is substantially undoped, and a fifth layer that is doped with donors.

CROSS REFERENCE TO RELATED APPLICATION

This application claims benefit of Provisional Application No.60/596,295 filed Sep. 14, 2005, the entire disclosure of which is herebyincorporated by reference herein.

BACKGROUND OF THE INVENTION

This invention relates to an avalanche photodiode (APD) structure, andmore particularly to an APD structure with high multiplication gain andlow excess multiplication noise.

Referring to FIG. 1, a typical electron-multiplying SACM (separateabsorption, charge, multiplication) APD has a p-doped anode and ann-doped cathode, and between the anode and cathode, in sequence, anabsorption layer, a charge layer and a multiplication layer. The APD isused in a reverse bias mode, with the anode of the diode connected tothe cathode (negative terminal) of a DC supply and the cathode of thediode connected to the anode (positive terminal) of the DC supply. Thematerial and thickness of the absorption layer (typically InGaAs fornear-infrared applications between 1000-1700 nm) is chosen so that thereis a relatively high probability that a photon in the desired wavelengthrange incident upon the APD will generate an electron/hole pair (highquantum efficiency). The macroscopic electric field in an APD junctionbiased for operation (i.e. the field relevant to the function of thedevice through its influence on the average motion and energy of chargecarriers) causes the hole to drift towards the anode and the electron todrift towards the cathode.

Generally, APDs to be operated in linear (proportional) mode aredesigned to preferentially avalanche one carrier type, because thiscondition minimizes the statistical variation of avalanche gain aroundits mean value (multiplication noise). The choice of preferred carriertype is usually dictated by selection of the multiplication layermaterial, as contrast between carrier ionization rates is a fundamentalmaterial property. For the purpose of the following discussion we shallassume that electrons are the preferred charge carriers unless thecontext indicates otherwise. It should be understood that the carrierroles can be reversed as dictated by the choice of multiplication layermaterials, with attendant reversal of the ordering of absorption,charge, and multiplication layers between the anode and cathode of theAPD.

A SACM APD is doped so that the macroscopic electric field in itsmultiplication layer is higher than in other depleted sections of theAPD. Carriers that drift through the multiplication layer are thereforeaccelerated to relatively high energy levels, and a small population ofunusually energetic carriers attain sufficient energy to impact-ionize.Secondary electron/hole pairs created by the collision of energeticcarriers with the lattice add to the current flowing in the junction,and are themselves accelerated by the electric field. Secondary holeswill drift towards the anode and secondary electrons will drift towardsthe cathode, possibly impact-ionizing themselves. In this manner, foreach primary electron that is photoelectrically generated in theabsorber layer, multiple secondary electrons are generated in themultiplication layer and collected at the anode of the DC supply; theaverage ratio of secondaries to primaries is the multiplication gain ofthe APD. In the limit of high ionization rates for both carrier types, apositive feedback condition can be established, and the APD will undergoavalanche breakdown. Avalanche breakdown renders the junctionconductive, and the multiplication gain essentially goes to infinity.

The propensity of a carrier to impact-ionize depends upon severalvariables, including its overlap with accessible states that mightparticipate in an ionizing collision, and their density. These factorsare jointly constrained by the carrier's energy and the band structureof the semiconductor involved; the existence of a band gap means thatbelow a certain threshold carrier energy, no accessible states willexist for an ionizing collision. Accordingly, different semiconductormaterials are characterized by different ionization threshold energies,which roughly track their band gap. Details of band structure andmaterial-dependent transport properties are also responsible forcontrast between the carrier ionization rates in the same material.

Since a carrier must accumulate kinetic energy to impact-ionize, andsince constant random scattering with phonons and other carriers acts todissipate accumulated energy, measurable impact-ionization does not turnon until the applied field is so high that carriers have a reasonableprobability of accumulating the necessary ionization threshold energybetween scattering events. Thus, a relatively high electric field (e.g.400 kV cm⁻¹ or higher) must be created in the multiplication layer toobtain useable avalanche gain. The field strength required depends uponthe material used (e.g. InAlAs) and factors affecting the scatteringrate (mean free path), such as lattice temperature and incidence ofscattering centers. The material of the absorber layer (typically InGaAsas mentioned above) may have a considerably lower breakdown field thanthat of the multiplication layer (e.g. about 250 kV cm⁻¹). The thicknessand doping of the charge layer are selected to allow the high field toexist in the multiplication layer while a substantially lower fieldexists in the absorber layer.

The characteristics of the multiplication layer that favor creation of asecondary electron/hole pair from an impact by an electron willgenerally also favor creation of a secondary electron/hole pair from animpact by a hole, although as noted above, the degree to which each typeof collision is favored may contrast.

The signal to noise ratio of an optical receiver based on an APD dependson the excess multiplication noise which in turn depends on the ratio ofhole and electron ionization rates in the multiplication layer.Therefore, for a given electron ionization rate, the excessmultiplication noise can be reduced by reducing the probability that animpact by a hole will create a secondary electron/hole pair.

The material that is most commonly used as an absorber in an APDdesigned for the near infrared between 1000-1700 nm isIn_(0.53)Ga_(0.47)As and the multiplication layer of such an APD is madeof compound semiconductors that are compatible withIn_(0.53)Ga_(0.47)As. Unfortunately, such avalanche photodiodes tend tooperate with high excess multiplication noise owing to the lack ofcontrast between electron- and hole-initiated impact ionization rates inthe materials involved. Therefore, efforts to construct avalanchephotodiodes capable of operating at high multiplication gain (M>>10)with low excess multiplication noise (k_(eff)<<0.4) in the near infraredhave not been entirely successful.

In the late 1980's and early 1990's, superlattice APDs were developedthat relied upon harvesting the potential energy drop associated withcarrier propagation over a band edge discontinuity. In certainsuperlattice material systems such asIn_(0.52)Al_(0.48)As/In_(0.53)Ga_(0.47)As, the band edge discontinuityis larger in one band (the conduction band) than the other, so onecarrier type (electrons) may tend to receive more of a boost in impactionization rate than the other. Recent academic criticism has called themechanism into question, and avalanche photodiodes with very high gainand low k_(eff) have not resulted.

In recent years, some researchers have demonstrated suppression ofexcess multiplication noise by a variety of techniques collectivelyknown as impact-ionization engineering (I²E). In its simplest form, I²Euses the dead space effect to limit the total number of differentionization chains that result from a carrier injected into a thin APDmultiplication layer. Dead space is the distance over which a coldcarrier must drift under the influence of the multiplying junction'smacroscopic electric field before it has picked up sufficient kineticenergy to initiate impact ionization. Dead space may represent asignificant fraction of the total volume inside a thin multiplicationlayer, in which case the spatial localization of impact ionization actsto create a correlation between ionization events that ultimately lowersthe multiplication noise of the APD.

A second I²E technique that has been described in the literature is theuse of well-and-barrier heterostructure multiplication regions in whichthe difference in impact ionization threshold energy between two or morematerials is exploited to enhance the impact ionization rate of onecarrier type over the other. Electrons drift in the opposite directionfrom holes because they have opposite charge; in a thin heterostructuremultiplication region with high threshold material at one end and lowthreshold material at the other, the ionization rate for the carriertype traveling from high threshold to low threshold will be enhanced,and that of the carrier type traveling from low threshold to highthreshold will be suppressed. The reason is that the low thresholdmaterial, in which ionization is easy, will be in the dead space of onetype, but not the other. A sharper contrast between ionization rateshelps to reduce the number of possible ionization chains by eliminatingsome of those involving feedback, and so the net result is lower excessmultiplication noise.

Thus far, it has not been possible to translate thelow-multiplication-noise operation of I²E APDs to high gain. Thislimitation is a consequence of the requirement that their multiplicationlayers be very thin in order to benefit from the dead-space effect. Athin multiplication layer has low noise because the number of possibleionization chains is small; by the same token, the long ionizationchains necessary to get high gain cannot fit inside a thinmultiplication layer. Higher gain can be eked out of a thinmultiplication layer by increasing the field strength, but in doing so,the contrast between ionization rates is lost, feedback is enhanced, andnoise suppression is lost precisely because a larger number ofionization chains can now fit into the same space. The stronger fieldsnot only degrade excess noise performance but also enhance dark currentleakage mechanisms such as band-to-band tunneling and thermionic fieldemission.

It might at first appear that if individual I²E multiplication layerscannot be operated at high gain and still preserve their low noisecharacter, it would be possible to achieve high gain by employingseveral multiplication layers each operating at low gain and cascadingthem in stages. However, simply growing a series of I²E multiplicationlayers is not sufficient: without some way to prevent feedback betweenstages, all that is obtained by stacking a series of thin multiplicationlayers is a single thick (and noisy) multiplication layer.

U.S. Pat. No. 6,747,296 B1 describes an avalanche photodiode with acascaded multiplication structure, with each stage of the multiplicationstructure being composed of multiple layers. Each multiplying stagecomprises (in the direction of travel of electrons, which are thepreferred charge carriers) a layer of a first material M1 and a layer ofa second material M2, the impact ionization threshold of the secondmaterial being lower than that of the first material. The layer of thefirst material includes (in succession, in the direction of travel ofelectrons) a first intrinsic region, a p-doped region, a secondintrinsic region, and an n-doped region. The layer of the secondmaterial is intrinsic. Thus, the placement of doping is such as to raisethe macroscopic electric field strength to its maximum in the layer ofmaterial having the higher ionization threshold, and to lower it withinthat layer. This multiplication structure is intended to result in theprobability of an ionizing impact being a maximum when an electronenters the material M2, where the field is lower than the maximum fieldin the layer of the material M1.

U.S. Pat. No. 6,747,296 B1 describes a hole step-down region inconnection with FIGS. 3A-3D. The function of the hole step-down regionis described in terms of band edge discontinuities. Thus, various formsof grading in composition are described as either (a) preventingnon-preferred charge carriers from harvesting band edge discontinuityenergy to enhance their impact ionization rate, or (b) facilitatingtransport of preferred charge carriers across the band discontinuity inorder to increase device speed. For this reason, U.S. Pat. No. 6,747,296B1 discloses that the material used in the hole step-down region must beof intermediate band gap. U.S. Pat. No. 6,747,296 B1 does not prescribeany requirements regarding the relative electric field strength in thehole step-down regions.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is providedan avalanche photodiode (APD) comprising an anode layer, a cathodelayer, an absorption layer between the anode layer and the cathodelayer, a first multiplying stage between the absorption layer and thecathode layer, a second multiplying stage between the first multiplyingstage and the cathode layer, and a carrier relaxation region between thefirst and second multiplying stages, wherein the macroscopic fieldstrength in the carrier relaxation region is lower than the macroscopicfield strength in the first and second multiplying stages, and thecarrier relaxation region is composed of a semiconductor material havinga fundamental band gap that is greater than or equal to the greatestfundamental band gap of semiconductor material in the first multiplyingstage and is greater than or equal to the greatest fundamental band gapof semiconductor material in the second multiplying stage.

According to a second aspect of the present invention there is providedan avalanche photodiode (APD) comprising an anode layer, a cathodelayer, an absorption layer between the anode layer and the cathodelayer, a first multiplying stage between the absorption layer and thecathode layer, a second multiplying stage between the first multiplyingstage and the cathode layer, and a carrier relaxation region between thefirst and second multiplying stages, and wherein each multiplying stagecomprises, in the direction of drift of electrons, a first layer that isdoped with acceptors, a second layer that is substantially undoped, athird layer that is doped with acceptors, a fourth layer that issubstantially undoped, and a fifth layer that is doped with donors.

All low-noise I²E multiplication regions depend on dead space in one wayor another to function. The present invention is based in part on therecognition that if a hot carrier arrives at a multiplying stage withsufficient kinetic energy to ionize, then it has no dead space withinthat stage, and the stage cannot function as designed. Accordingly, whenI²E multiplying stages are cascaded, they must be separated by carrierrelaxation regions in which hot carriers can lose their energy,effectively resetting their dead space. Hot carriers normally lose theirenergy through collisions with other carriers and phonons, so that evenunder very high macroscopic electric fields, they will tend to have amaximum average velocity that is near the thermal velocity: this isknown as the saturation velocity. However, as carrier scattering is arandom process, under extremely high fields a portion of the carriersare able to accelerate far above the saturation velocity and attainsufficient kinetic energy to impact ionize. This is why APDmultiplication regions require high internal fields to function. Simplyby dropping the macroscopic electric field strength, the population ofactive carriers can be rapidly depleted through normal scatteringmechanisms.

Manipulation of the macroscopic electric field can be used for anadditional purpose. Empirical measurements have found that impactionization rates are an exponential function of the macroscopic electricfield strength. Accordingly, differences in macroscopic electric fieldstrength can be used to enhance and suppress ionization rate. Thismechanism of noise suppression through field control is a form of bandengineering that can be implemented through doping during epitaxialgrowth.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show how the samemay be carried into effect, reference will now be made, by way ofexample, to the accompanying drawings, in which:

FIG. 1 is a simplified schematic view of a conventional SACM APD,

FIG. 2 is a simplified sectional view of an SACM APD embodying thepresent invention,

FIG. 3 is a more detailed view of one of the multiplying stages of theAPD shown in FIG. 2,

FIG. 4 is a graph illustrating both the electric field profile and theionization threshold profile in the multiplying stage, and

FIG. 5 is a schematic sectional view of an SACM APD embodying thepresent invention.

DETAILED DESCRIPTION

An embodiment of the present invention addresses the limited gain of I²Emultiplication layers by showing how they can be deployed to achievehigh gain yet still retain their low noise properties.

The APD shown schematically in FIG. 2 comprises an absorption layer 10,a charge layer 12 and a multiplication layer 14 produced by molecularbeam epitaxy on a substrate 16 of InP. The multiplication layercomprises several (nine, for example) repetitions of a multilayersequence 18, and a sequence of transition layers 20, 22 and 24 betweenthe last repetition of the sequence 18 and the n+ layer. FIG. 3 showsthe last two repetitions of the six-layer sequence and the threetransition layers.

Referring to FIG. 3, the multiplication layer and the sequence oftransition layers form ten cascaded multiplying stages, each having fourdistinct functional layers: a first p-doped field-up layer 30, a firstintrinsic layer or plateau 32, a second p-doped field-up layer 34, asecond intrinsic layer or plateau 36, an n-doped field-down layer 38,and an intrinsic, i.e. undoped, relaxation layer 40. In the firstrepetition of the sequence, the charge layer 12 plays the role of thefirst field-up layer 30 and the first intrinsic layer or plateau 32. Thefirst field-up layer raises the electric field in the first plateau 32,the second field-up layer raises the electric field in the secondplateau 36, and the field-down layer lowers the electric field in therelaxation layer 40. The doping profile therefore results in an electricfield profile in each multiplying stage, traversed in the direction ofdrift of electrons, by which the field increases substantially in thefirst field-up layer 30, remains constant in the first plateau 32,increases substantially in the second field-up layer 34, remainsconstant in the second plateau 36, falls abruptly in the field-downlayer 38, and remains constant in the relaxation layer 40, as shown inFIG. 4. As electrons traverse the multiplying stage, they areaccelerated by the field in the first field-up layer 30, the firstplateau 32 and the second field-up layer 34. The doping levels anddimensions of these layers are specified so that the electric field inthe first plateau 32 is too low to sustain a high rate of carrierionization. At the same time, the electric field engineered by thedoping pattern is designed to insure that once electrons transit thefirst plateau 32 and the second field-up layer 34, the high-energyportion of the electron population traveling above the saturationvelocity arrives at the second plateau 36 with sufficient energy toionize readily. The macroscopic electric field reaches its maximum inthe second plateau 36, or ionization layer. Accordingly, the impactionization rate also reaches a maximum in the ionization layer. In orderto take maximum advantage of the increase in ionization rate due to thehigh electric field, the ionization layer is made of a material having alower band gap (and consequently lower ionization threshold) than thematerials used for the other layers of the multiplying stage. Theionization layer is sized so that any secondary holes created by thedesired impact ionization can't pick up sufficient energy to causefurther ionizations before they drift out of the high-field region. Inthis way, electrons are encouraged to ionize preferentially in thesecond plateau 36, but not in the preceding layers of the multiplyingstage, and without feedback. Electrons next encounter the field-downlayer 38, in which the electric field strength drops rapidly below thelevel required to sustain impact ionization, returning to the value atthe beginning of the multiplying stage. From there the electrons driftinto the low-field carrier relaxation layer 40, where normal scatteringprocesses equilibrate any remaining high-energy electrons with thegeneral population. Accordingly, when the electrons arrive at the firstfield-up layer 30 of the next multiplying stage, their energy isrelatively low.

A population of holes will drift through the multiplying stage in theopposite direction, and they will therefore encounter its layers in theopposite order. Layers that raise the electric field when encounteredfrom the left lower it when encountered from the right, and vice versa,so the descriptive terms “field-up” and “field-down” are omitted herefor clarity. Holes entering the stage through the carrier relaxationlayer 40 arrive at ionization layer 36 with insufficient accumulatedkinetic energy to ionize, despite the high field and low ionizationthreshold in that layer. The distance over which the holes have traveledin the increasing field is substantially less than that traveled byelectrons in the opposite direction, so the total potential drop theyencounter is smaller. The sudden increase in the electric field whichtakes place across layer 38 acts to increase the energy of the holepopulation, but layer 38 is sized so that the total potential drop isinsufficient to boost holes above the ionization threshold energy. Theholes therefore transit ionization layer 36 with a much lowerprobability of ionizing than the equivalent population of electrons,because they start with less energy, even though they continue toaccumulate kinetic energy in the ionization layer's high electric field.The vast majority of holes enter layer 34 without having impact ionized.A small portion of high-energy holes may ionize outside the ionizationlayer, but the higher ionization threshold minimizes this, as does theirrapid loss of energy through scattering in the successively lower fieldsof layers 34, 32 and 30. Ultimately most holes enter the relaxationlayer 40 of the next multiplying stage without having created anelectron/hole pair. In the relaxation layer 40 of the next stage, theenergy gained from the field in the preceding stage is lost incollisions. Accordingly, when the holes arrive at layer 38 of the nextmultiplying stage their average energy is relatively low, and theprocess can repeat.

Thus, for each multiplying stage there is a high probability that anelectron entering the second plateau will create an electron/hole pairand a small probability that a hole traversing the multiplying stagewill create an electron/hole pair. Accordingly, a high multiplicationgain is achieved without a commensurate increase in the excessmultiplication noise associated with feedback.

The present invention thus distinguishes from the conventionalsuperlattice APD by preventing, or at least reducing, feedback betweenmultiplying stages.

The macroscopic electric field in the carrier relaxation layer and thelayer's thickness are chosen so that carriers that have gainedsubstantial kinetic energy from the macroscopic electric field of onemultiplying stage will, with high probability, lose that accumulatedkinetic energy through random collisions prior to entering thesubsequent multiplying stage. In this manner, the carrier relaxationlayers suppress feedback (and the associated multiplication noise)between multiplying stages by preventing “active” carriers fromretaining kinetic energy between multiplying stages. Monte Carlo carriertransport simulations have shown that active carriers will only retaintheir energy over roughly 30 nm. However, a large proportion of carrierswill lose sufficient energy to prevent ionization in the nextmultiplying stage if the carrier relaxation layer is about 20 nm inlength. Thus, the carrier relaxation layers should be at least about 20nm in length, although it is preferred that they be at least 30 nm inlength and in the preferred embodiment of the invention they are 100 nmin length.

The final repetition 189 of the multilayer sequence is shared betweenthe final complete multiplying stage 44 and a partial multiplying stage46 that does not include a carrier relaxation layer. Electrons thatleave the carrier relaxation layer 40 of the multiplying stage 44traverse the succeeding layers 30 and 32 and enter the transition layer20, which is a final field-up layer leading to a final ionization layer22. High energy electrons that enter the layer 22 may impact ionize, asdescribed above. The layer 24 is a final field-down layer lowering theelectric field in the n⁺ layer 50. There is no need to provide a carrierrelaxation layer after the field-down layer 24, because there is noionization layer to supply holes that would enter the field-down layer24.

The multiplying stages that are cascaded by means of the relaxationlayers described with reference to FIGS. 2-4 are of theimpact-ionization-engineered variety, but use of the relaxation layersis not confined to the specific multiplying stages described withreference to FIGS. 2-4 and they may be applied to multiplication stagesemploying different mechanisms, such as the barrier-and-well multiplyingstages described in U.S. Patent Application 20030047752. Themultiplication stage described with reference to FIGS. 2-4 differs fromthat disclosed in U.S. Patent Application 20030047752, in thatmacroscopic electric field strength is used in addition to materialselection to raise and lower the probability of impact ionization.

FIG. 4 shows that the band gap in the ionization layer 36 is somewhatless than in the other layers of the multiplying stage. The narrow bandgap in the ionization layer provides the desired contrast in ionizationthreshold, as explained above. Since the function of the relaxationlayer 40 depends only on the reduction in electric field strength andthe physical thickness of the layer, and does not require a small bandgap, the band gap in the relaxation layer 40 should be at least as greatas that in other layers of the multiplying stage in order to minimizedark current leakage.

FIG. 5 illustrates a preferred SACM APD embodying the present invention.

Referring to FIG. 5, the layer 50 is a p-contact layer having a narrowband gap near the surface and a wide gap below the surface. The layer 50provides a low-resistance electrical contact between the diode and themetallurgical contact and forms the p-side of the diode.

The absorber layer 52 is an intrinsic layer having a narrow band gap.The layer 52 efficiently absorbs the optical signal and generatesphotocarriers.

The charge layer 54 is lightly p-type having a wide band gap, and servesto regulate the electric field in the absorber layer and to suppressabsorber leakage. It also plays the role of the first field-up andplateau layers of the very first multiplication stage.

The first field-up layer 30 is lightly p-type and has a wide band gap.The layer 30 elevates the electric field in the layer 32 relative tothat in the carrier relaxation layer 40.

The plateau, or carrier heating layer, 32 is an intrinsic layer having awide band gap. The layer 32 provides a layer in which the kinetic energyof electrons can increase.

The second field-up layer 34 is lightly p-type and has a wide band gap.The layer 34 elevates the electric field in the layer 36 relative tothat in the plateau 32.

The ionization layer 36 is an intrinsic layer having a medium band gap,which is less than that of the layer 34. The ionization layer promotesimpact ionization by electrons.

The field-down layer 38 is lightly n-type and has a wide band gap. Thelayer 38 lowers the electric field in the layer 40 relative to that inthe layer 36.

The carrier relaxation layer 40 is an intrinsic layer having a wide bandgap. The layer 40 allows hot holes to lose their kinetic energy in a lowfield layer, thereby suppress feedback between successive multiplyingstages.

The layer 56 forms the n-side of the diode. In this specific embodiment,metallurgical electrical contact would be made to the conductive n-typeInP substrate. It should be appreciated than an equivalent structurecould be grown on a semi-insulating InP substrate, and metallurgicalcontact made to layer 56 directly.

The layers that are between the layers specifically identified above arespecified to improve the quality of growth as well as reduce theelectrical resistance of portions of the structure.

The present invention may be used to provide a technique for suppressingcarrier feedback between cascaded multiplying stages inside an avalanchephotodiode (APD) multiplication layer. When operated at a given averagemultiplication gain, an APD fabricated with a plurality of cascadedlow-noise multiplying stages will operate with lower excessmultiplication noise and lower dark current leakage than a substantiallyequivalent APD fabricated with a single such multiplying stage. In anembodiment of the invention, each multiplying stage is separated fromits neighbors by carrier relaxation layers characterized by lowmacroscopic electric field strength. The carrier relaxation layers aredesigned such that carriers that have gained substantial kinetic energyfrom the macroscopic electric field of one multiplying stage will, withhigh probability, lose that accumulated kinetic energy prior to enteringthe subsequent multiplying stage.

The term “intrinsic” is commonly used in connection with semiconductormaterial to indicate that the material is not doped. However, use of theterm “substantially undoped” or “intrinsic” or the abbreviation “i” inthe description and the appended claims is not intended to suggest orrequire that the material is devoid of dopants and, in particular, isintended to cover the possibility of the material being unintentionallydoped.

It will be appreciated that the invention is not restricted to theparticular embodiment that has been described, and that variations maybe made therein without departing from the scope of the invention asdefined in the appended claims and equivalents thereof. For example,although the multiplication layer 14 of the APD shown in FIG. 2 includesnine identical multilayer sequences 18, and accordingly the average gainprovided by each multiplying stage is the same as that provided by theother multiplying stages, this is at least partially for ease ofmanufacture and the invention includes within its scope an APD in whichone or more of the multiplying stages provides a different average gainfrom other stage(s) or indeed each multiplying stage provides adifferent average gain from each of the other stages. Unless the contextindicates otherwise, a reference in a claim to the number of instancesof an element, be it a reference to one instance or more than oneinstance, requires at least the stated number of instances of theelement but is not intended to exclude from the scope of the claim astructure or method having more instances of that element than stated.If the word “comprises” or “includes,” or a derivative of either ofthese words is used in this specification, including the claims, it isused in an inclusive, not exclusive or exhaustive, sense. Thus, forexample, a statement that a component comprises first and secondelements is not intended to exclude the possibility of the componentincluding one or more additional elements.

1. A two-terminal avalanche photodiode (APD) comprising: first and second electrode layers, one of said first and second electrode layers being an anode layer and the other of said first and second electrode layers being a cathode layer, an absorption layer between the first and second electrode layers, a first multiplying stage between the absorption layer and the first electrode layer, a second multiplying stage between the first multiplying stage and the first electrode layer, wherein the electrical potential across each stage is established by the selection of doping levels in the structure and the voltage applied across the first and second electrode layers only, so that more than one multiplying stage can be operated without making additional independent electrical contacts to the APD, and a carrier relaxation region between the first and second multiplying stages to which no direct external electrical contact is made, wherein the macroscopic field strength in the carrier relaxation region is lower than the macroscopic field strength in the first and second multiplying stages, and the carrier relaxation region is composed of an undoped semiconductor material having a fundamental band gap that is greater than or equal to the greatest fundamental band gap of semiconductor material in the first multiplying stage and is greater than or equal to the greatest fundamental band gap of semiconductor material in the second multiplying stage.
 2. An APD according to claim 1, wherein at least one of the multiplying stages comprises, in the direction of drift of preferred charge carriers of one polarity, a first layer that is doped with dopant that generates free charge carriers of opposite polarity to said one polarity, a second layer that is substantially undoped, a third layer that is doped with dopant that generates free charge carriers of said opposite polarity, a fourth layer that is substantially undoped, and a fifth layer that is doped with dopant that generates free charge carriers of said one polarity.
 3. An APD according to claim 2, wherein the fourth layer comprises a semiconductor of fundamental band gap less than that of the materials of the third and fifth layers.
 4. An APD according to claim 1, wherein at least one of said multiplying stages comprises, in the direction of drift of electrons, a first layer that is doped with acceptors, a second layer that is substantially undoped, a third layer that is doped with acceptors, a fourth layer that is substantially undoped, and a fifth layer that is doped with donors.
 5. An APD according to claim 1, wherein the carrier relaxation region has a thickness greater than 20 nm.
 6. An APD according to claim 1, comprising at least one additional multiplying stage between the second multiplying stage and the first electrode layer and, for each additional multiplying stage, an additional carrier relaxation region between the second multiplying stage and said additional multiplying stage, and wherein each carrier relaxation region is located between two consecutive multiplying stages in the direction of drift of preferred charge carriers, and each additional multiplying stage is located either between two consecutive carrier relaxation regions in said direction or between a carrier relaxation region and the first electrode layer.
 7. An APD according to claim 1, wherein the first electrode layer is the cathode layer.
 8. An avalanche photodiode (APD) comprising: an anode layer, a cathode layer, an absorption layer between the anode layer and the cathode layer, a first multiplying stage between the absorption layer and the cathode layer, a second multiplying stage between the first multiplying stage and the cathode layer, and a carrier relaxation region between the first and second multiplying stages, and wherein each multiplying stage comprises, in the direction of drift of electrons, a first layer that is doped with acceptors, a second layer that is substantially undoped, a third layer that is doped with acceptors, a fourth layer that is substantially undoped, and a fifth layer that is doped with donors.
 9. An APD according to claim 8, wherein the fourth layer comprises a semiconductor of fundamental band gap less than that of the materials of the third and fifth layers.
 10. An APD according to claim 8, wherein the second layer of a given multiplying stage comprises a material having a impact ionization threshold energy greater than or equal to that of the material of the fourth layer of that multiplying stage.
 11. An APD according to claim 8, wherein the carrier relaxation region comprises a material with impact ionization threshold energy greater than or equal to that of the material of the second layer of each of the first and second multiplying stages.
 12. An APD according to claim 8, comprising at least a third multiplying stage between the second multiplying stage and the cathode layer, and a second carrier relaxation region between the second and third multiplying stages, and wherein the second carrier relaxation region comprises a material with impact ionization threshold energy greater than or equal to that of the material of the second layer of each of the second and third multiplying stages.
 13. An APD according to claim 8, wherein the carrier relaxation region has a thickness greater than 20 nm.
 14. An APD according to claim 8, comprising at least one additional multiplying stage between the second multiplying stage and the cathode layer and, for each additional multiplying stage, an additional carrier relaxation region between the second multiplying stage and said additional multiplying stage, and wherein each carrier relaxation region is located between two consecutive multiplying stages in the direction of drift of electrons, and each additional multiplying stage is located either between two consecutive carrier relaxation regions in said direction or between a carrier relaxation region and the cathode layer. 